In the field of animal husbandry, it has been recognized that farrowing stalls and nursery pens, as illustrated in U.S. Pat. No. 4,348,986, reduce labor costs and increase livestock production. Such farrowing stalls and nursery pens typically include a raised, heavy gauge, wire mesh flooring supporting the associated stall or pen. These structures, however, lead to waste disposal problems. Accordingly, there have been developed in the prior art flush trays for use in conjunction with such farrowing stalls and nursery pens and in conjunction with other animal husbandry applications. Such trays, because of the applications to which they are put, preferably include projecting supporting ribs formed as a portion thereof.
However, the prior trays suffer from disadvantages resulting from the method of manufacture of the same. For example, where a prior art flush tray is manufactured by a vacuum molding process, the product is subject to stress cracks and breakage.
One significant reason for such difficulties in the prior art is a difficulty in controlling the thickness of articles formed by the vacuum molding process. Rotational molding provides improvements in the properties both of the plastic used and of the resulting product, and results in stress-free articles having predictable wall thickness and in substantial elimination of waste of raw material. However, a disadvantage of such a procedure exists with respect to large articles of manufacture.
As has been described in U.S. Pat. No. 4,285,903, the rotational molding process provides a predetermined quantity of resinous material to a mold which is then rotated to distribute the material for solidification along the surface of the mold. However, for large articles there arises a problem in view of the use of quick setting resins. Particularly, as disclosed therein, the resin beings to set prior to reaching all portions of the mold, so that a nonuniform, lumpy, or otherwise uneven structure results. The U.S. Pat. No. 4,285,903 recognizes the problem and provides a complex, closed loop, programmed control apparatus in a solution which introduces the molding material to the mold at a plurality of locations. Thus, the disclosed control apparatus is intended to distribute the molding material throughout the mold before the material has set, or begun to set, to shape.
Another patent disclosure recognizes the problem of providing uniform wall thickness for rotationally molded products. U.S. Pat. No. 4,146,565 discloses a progressively decreasing rate of motion for the mold, in accordance with the increase of viscosity of material being molded, in order to provide the desired uniform wall thickness. However, similarly to the U.S. Pat. No. 4,285,903 disclosure, the suggested solution of U.S. Pat. No. 4,146,565 requires complex and expensive redesign, specifically of specialized molding equipment.
U.S. Pat. No. 4,158,686 teaches the use of a rotational molding apparatus for formation of an article having stemlike projections F jutting out therefrom. However, no recognition is found therein of any difficulties caused by the projections or the rotational molding process.
Accordingly, there is a need in the prior art for method or apparatus for providing large scale articles of manufacture which are substantially stress- or crack-free, specifically for formation of such articles by a rotational molding process. However, there is also a need for provision of substantially uniform wall thickness for large scale products such as flush trays, and particularly for such products having ribs projecting therefrom.